Although metal nanoparticles are widely used in electrocatalysis, in particular for the preparation of the electrodes for fuel cells, it is still not fully understood how the size of a metal phase affects its electrochemical reactivity. In this paper, we demonstrate a possible approach to design model electrodes for studying size effects in electrocatalysis. A simple chemical deposition procedure is introduced, which allows reproducible preparation of 1.5/3.0 nm Pt nanoparticles anchored to the surface of glassy carbon (GC) or highly oriented pyrolytic graphite (HOPG). Pt nanoparticles supported on GC are stable versus potential variation and can be used for studying size effects. They are examined in electrochemical reactions relevant to low temperature fuel cells: electrooxidation of adsorbed CO and methanol. It is demonstrated that reactivity of Pt nanoparticles in these reactions is considerably decreased in comparison to bulk polycrystalline Pt. (C) 2003 Elsevier Ltd. All rights reserved.